1. Field
This invention relates generally to radio frequency communications and, more specifically, to devices and circuits for generating signals to be used for frequency conversion in image-reject mixers.
2. Description of Related Art
To implement low-cost single-chip radio frequency (RF) transceivers, current architectures employ zero intermediate-frequency (zero-IF) or low-IF techniques. In such approaches, image frequency signals are not suppressed by filtering as was the case with previous heterodyne architectures. In current zero-IF and/or low-IF architectures, image-reject or quadrature up-conversion and down-conversion mixers are used. One drawback of such image-rejection transceivers is that they rely on a local oscillators (LOs) that have an in-phase component (I) and a quadrature component (Q) that are phase spaced exactly 90° apart. As a result of this phase relationship, the I and Q LO signals often cause functional difficulties in practical implementations.
For example, in one such embodiment the LO signals in quadrature are generated by a 4-stage ring oscillator. Most ring oscillator implementations, however, generate a significant enough amount of phase noise that they are impracticable for use in most telecommunications applications. Alternatively, the LO signals (I and Q) generation can be generated from a signal that is generated by a voltage controller oscillator (VCO) coupled with a resistive-capacitive/capacitive-resistive (RC/CR) circuit. In such an embodiment, the performance of the circuit is dependent on the RC time constant of the RC/CR circuit being equal to the input signal period. Such a relationship is difficult to achieve due to normal manufacturing variation of resistors and capacitors used in such an RC/CR circuit. Furthermore, in such embodiments, the oscillator typically runs at the same frequency as the RF output signal, which results in the VCO being susceptible to pulling by (interference from) the RF power amplifier. This pulling results in unwanted spurious variations in the output signal of such circuits.
Another approach that has been implemented is the use of a quadrature inductive-capacitive (LC) oscillator that includes two high-quality LC oscillators coupled in such a way that their output signals are spaced 90° apart. However, for such embodiments, the two LC oscillators take up a large area (such as in an integrated circuit) and they are also very susceptible to pulling, as was described above.
Still another approach that could be employed is to use a local oscillator that operates at twice the frequency of the output signal. In such an implementation, signals in quadrature can be obtained with a simple divide-by-2 toggle flip-flop. However, an oscillator operating at twice the operating frequency (e.g., radio frequency) of the output signals, even if possible notwithstanding technological limitations, would consume a large amount of power, which is undesirable. Furthermore, such an approach would not adequately resolve the pulling problem either, as the VCO could still be pulled by the second harmonic of the power amplifier.